1. Field of the Invention
The invention relates to a lighting system comprising
a low-pressure discharge lamp provided with a discharge vessel which is closed in a gastight manner and contains an ionizable filling, which low-pressure discharge lamp is further provided with electrodes for maintaining an electric discharge in the discharge vessel, which electrodes are provided with a first electrode body of a refractory metal which is electrically connected to current conductors which extend from the interior of the discharge vessel to the exterior, which first electrode body is at least partly covered with an electron-emitting material, and
a ballast circuit, which is connected to the current conductors, and which serves to cold ignite the low-pressure discharge lamp and to feed a high-frequency lamp current to the low-pressure discharge lamp.
2. Description of the Prior Art
Such a lighting system is disclosed in EP 0491420 A1. As a result of the cold ignition of the low-pressure discharge lamp by the ballast circuit, the low-pressure discharge lamp ignites very rapidly and the ballast circuit is comparatively simple and hence inexpensive. A drawback of the cold ignition of the low-pressure discharge lamp resides in that the switching performance, i.e. the number of times that the low-pressure discharge lamp can be switched on is relatively small. This is caused by the fact that during igniting a glow discharge develops, under the influence of the high-frequency voltage across the low-pressure discharge lamp, which glow discharge strikes only a part of the electrode body. This electrode body is generally formed in practice by a double spiral. As a result, this part of the electrode body is so intensively bombarded with positive ions that sputtering of the refractory metal takes place. This sputtering eventually causes a break in the electrode body, which means the end of the service life of the low-pressure discharge lamp.
It is an object of the invention to provide a lighting system wherein the low-pressure discharge lamp is cold ignited by the ballast circuit, and the switching performance of the low-pressure discharge lamp is comparatively high.
To achieve this, a lighting system as mentioned in the opening paragraph is characterized in accordance with the invention in that at least one of the electrodes is provided with a second electrode body of a refractory metal, which is electrically connected to the current conductors and which is at least partly covered with an electron-emitting material.
The presence of the second body enables the total amount of electron-emitting material necessary to maintain the discharge during stationary lamp operation to be distributed over the two electrode bodies. As a result, the temperature distribution over the electrode bodies during igniting the low-pressure discharge lamp is much more homogeneous than the temperature distribution over the electrode body of the electrode in the low-pressure discharge lamp of the known lighting system. By virtue of this more homogeneous temperature distribution, the glow discharge, which occurs during the ignition, does not always strike at the same location. As a result, also the sputtering of refractory material is more evenly distributed over the electrode (or over the electrode bodies). By virtue of the latter, a break in the electrode only occurs after the low-pressure discharge lamp has been ignited a comparatively large number of times.
In most cases, the low-pressure discharge lamp is provided with two electrodes. In order to extend the service life of each of these electrodes, each electrode is preferably provided with a second electrode body of a refractory metal, which is electrically connected to the current conductors and at least partly covered with an electron-emitting material.
In some cases it may be desirable to further improve the homogeneity of the temperature distribution by providing at least one of the electrodes with three or more electrode bodies of a refractory metal, which are electrically connected to the current conductors and at least partly covered with an electron-emitting material.
Good results have been achieved with lighting systems in accordance with the invention wherein each of the electrode bodies connects two current conductors with each other. The electrode bodies are connected in parallel. Apart from a more homogeneous temperature distribution, it is achieved that, in case of breakage of one of the electrode bodies, at least one electrode body remains, so that the electrode can remain functional.
Good results have also been achieved with embodiments of a lighting system in accordance with the invention wherein the first and the second electrode body are each formed by a double spiral of refractory material, which is at least partly covered with an electron-emitting material. This double spiral is formed by winding a wire, wound in the form of a first spiral (the primary winding), into the form of a second spiral (the secondary winding). More particularly it has been found that a good homogeneity of the temperature distribution can be brought about if the secondary winding of the double spiral of each electrode body comprises at least 8 turns. Since there are at least 8 turns between two current conductors, the temperature distribution over these 8 turns is very homogeneous.
In a preferred embodiment of a lighting system in accordance with the invention, the lighting system is a compact fluorescent lamp comprising a housing which is secured to the low-pressure discharge lamp, the ionizable filling containing mercury and at least an inert gas, the discharge vessel being provided at an inner surface with a luminescent layer, the housing also being secured to a lamp cap having contacts, and the ballast circuit being accommodated in the housing and being electrically connected to the contacts and the current conductors.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.